External disk not usable anymore after Monterey beta update/install

During update to Monterey beta to an external ssd, startup stopped with barred circle and unusable ssd!

I have a recent time machine backup, still I don't know how to repair the respective container. In diskutil the volume is shown as "AppleAPFSMedia not initialized" and can not be repaired!

'diskutil apfs list' shows an error (Container ERROR -69808).  See partial listing:

APFS Containers (3 found)
|
.
.
.
|
+-- Container ERROR -69808
    ======================
    APFS Container Reference:     disk8
    Size (Capacity Ceiling):      ERROR -69620
    Capacity In Use By Volumes:   ERROR -69620
    Capacity Not Allocated:       ERROR -69620
    |
    +-< Physical Store disk3s2 1C0E6E67-8751-4376-8C2E-9DAA6460C864
    |   -----------------------------------------------------------
    |   APFS Physical Store Disk:   disk3s2
    |   Size:                       599995129856 B (600.0 GB)
    |
    +-> No Volumes

External disk not usable anymore after Monterey beta update/install
 
 
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